Power line filters are commonly included in electronic equipment where protection is required against potentially damaging transient voltages on the power supply lines. In addition, such filters are frequently required to prevent the passage inward and outward of a wide range of signal frequencies, present at levels below the threshold of damage but sufficiently energetic to cause interference with proper function of the protected equipment, or other equipment sharing the same power line. Electrically, a filter consists of some number of capacitive and inductive elements arranged in a low-pass configuration, capable of passing the required supply current but providing attenuation of all unwanted frequencies and "spike" signals. A variety of power-line filters are available, and their use is widespread.
A serious shortcoming of the filters emerges when their conditions of application are examined. It is a commonplace of filter design that the required filter performance will not be met unless the design assumptions of source and termination impedance are closely approached in actual application. Values of 50-ohm source and termination impedance are often assumed in the design of line filters, and are stipulated by MIL-STD-220A for line filter testing. By contrast with this assumption, termination impedance is decided by the load which the filter feeds and may vary widely between near-short and near-open circuit values; source impedance is decided in the power line, and both in formal test and in the real world may be very low. The resulting filter performance may depart widely from design intent, and can in commonly-encountered conditions become worse than useless. Given low source and high termination impedance, the filter becomes a voltage amplifier which actually enhances high-voltage transients. Commercially available filters tested under these conditions have been found to have voltage gains as high as 60 times from input to output for certain signal frequencies.